It is of primary importance for the operation of an elevator system that the system should work correctly and above all safely.
For this reason, elevator systems employ a number of different safety devices. One of these is the so-called safety circuit. The safety circuit is the most important part of the electric safety system of an elevator. The safety circuit extends in the elevator shaft from one safety device to another. The circuit typically consists of safety device contacts and switches chained in series. If any one of the safety devices interrupts the safety circuit, the elevator will stop or will not start moving. The safety circuit monitors e.g. the car doors, landing doors, locks, etc. For example, if the doors of the elevator car are open, then the safety circuit is open and the elevator should not start moving under any circumstances.
An elevator in use must be maintained and its condition must be statutorily checked to guarantee its safe operation. To check the condition of an elevator, it is subjected to operation tests, in other words, the operation of the safety and alarm equipment is tested and checks are carried out to make sure that the elevator does not move before the car and landing doors are closed and that the doors do not open before the elevator is at a floor. In the condition monitoring inspection, it is possible to use various condition monitoring equipments, including analyzers that can utilize information regarding the current flowing in the safety circuit.
It is thus possible to make inferences about the condition of the elevator by observing the operation of the safety circuit. Based on the intensity of the current flowing in different parts of the safety circuit, it is possible to infer which ones of the switches comprised in the safety circuit are closed at each instant of time and whether the elevator is functioning in accordance with the regulations imposed on it.
However, measuring the current from intermediate taps of the safety circuit is problematic because there are regulatory restrictions on the right to touch the safety circuit. Changes concerning the safety circuit must always be submitted to authorities for approval, which is why measuring the current of the safety circuit is in itself difficult. Moreover, it may be difficult to measure the current from different points of the safety circuit because the switches of different parts of the safety circuit are located in the elevator shaft at a considerable distance from each other in regard of measuring technics.